(1) Field of the Invention
The present invention relates to a two-mode demodulating apparatus suitable for use in a radio terminal in, for example, a mobile communication system.
(2) Description of Related Art
Shortage of frequency bands usable as transmission frequencies with an increase of subscribers is showing up in recent radio communication system, which causes troubles in communication such that a telephone line is often interrupted or a telephone communication becomes broken, etc. In order to avoid such troubles in the communication, there have been developed and operated various techniques of improving efficiency of frequency utilization as countermeasures.
A digital communication system having a higher efficiency of frequency utilization using a linear modulation system comes to be used, for instance. However, the number of base stations to which such digital communication system having a higher efficiency of frequency utilization is still insufficient and an area in which the digital communication can be used is limited, as compared with analog communication systems using known linear modulating systems.
There is a demand for a communication terminal which can use both of the above two communication system to make a communication in a digital communication system in an area in which the digital communication system is usable, while making a communication in an analog system in an area in which the digital communication system is not usable but only an analog communication system is usable.
FIG. 12 is a block diagram showing a two-mode demodulating apparatus used as a receiving unit of a communication terminal being able to use two communication systems as above. Namely, a two-mode receiving apparatus 100 shown in FIG. 12 has a linear wave receiving circuit 110, a non-linear wave receiving circuit 120 and a digital processing unit 130.
In the two-mode receiving apparatus 100 shown in FIG. 12, the digital processing unit 130 can control the linear wave receiving circuit 110 to receive a linear modulated wave signal, and the non-linear wave receiving circuit 120 to receive a non-linear modulated wave signal of intermediate frequency (IF) signals as received signals.
The linear wave receiving circuit 110 has a variable gain amplifier 111, multipliers 112I and 112Q, a 90.degree. phase shifter 114, linear wave receiving band-limit filters 115I and 115Q, high-speed A/D (Analog/Digital) converters 116I and 116Q, a frequency finely tunable temperature-compensated oscillator (VC-TCXO) 117, and a PLL (Phase Locked Loop) unit 118.
The non-linear wave receiving circuit 120 has a multiplier 121, a local oscillator 122, a non-linear wave receiving filter 123, a limiter-amplifier 124, and a quadrature detector 125, a non-linear receiving filter 126 and an A/D converter 127.
The digital processing unit 130 has a liner wave receiving process unit 131, a local oscillated frequency setting unit 132, a frequency correcting unit 133, a gain controlling unit 134, a received electric field strength computing unit 135, and a non-linear wave receiving process unit 136.
In the two-mode receiving apparatus 100 with the above structure shown in FIG. 12, a linear modulated wave signal as a received signal received by the linear wave receiving circuit 110 is subjected to an automatic gain control in the variable gain amplifier 111, mixed with a local signal fed from the PLL unit 118 to be detected in orthogoanl detection in a quasi-synchronous system, whereby baseband signals in two systems are outputted.
The above PLL unit 118 performs a PLL control on a signal from the temperature-compensated oscillator 117, which has been subjected to a frequency control (quasi-synchronous correction) on the basis of the received signal by the digital processing unit 130, and outputs the signal as a local signal for the orthogonal detection in the multipliers 112I and 112Q.
The baseband signals (analog signals) outputted from the multipliers 112I and 112Q are band-limited in the respective linear wave receiving band-limit filters 115I and 115Q, converted and demodulated into digital signals in the respective high-speed A/D converters 116I and 116Q, and outputted to the digital processing unit 130 in the following stage.
A non-linear modulated wave signal as the receive signal received by the non-linear wave receiving circuit 120 is mixed with a local signal from the local oscillator 122 provided separately from the function unit (refer to reference numerals 117 and 118) local oscillator 118 outputting the above local signal for receiving a linear modulated wave in the multiplier 121, and converted into an intermediate frequency signal.
The intermediate frequency signal from the multiplier 121 is limited to a band set in advance by the non-linear wave receiving band-limit filter 123. In other words, the intermediate frequency signal having passed through the non-linear wave receiving band-limit filter 123 is limited to a signal whose band is fixedly set in advance.
The baseband signal having passed through the non-linear receiving band-limit filter 123 is limited and amplified by the limiter-amplifier 124, then subjected to a quadrature detection in the quadrature detector 125 configured with a multiplier 125a and a phase shifter 125b. In the non-linear wave receiving band-limit filter 126 and the A/D converter 127, only a desired modulated signal is selectively taken out from the baseband signal, converted into a digital signal, and outputted to the digital processing unit 130 in the following stage.
However, in the above two-mode receiving apparatus 100 shown in FIG. 12, the non-linear wave receiving band-limit filter 123 in, for example, the non-linear wave receiving circuit 120 is of a large size since it is a passive component applied a resonance phenomenon, resulting in a large scale circuit. If the above two-mode receiving apparatus is applied to a mobile terminal in a mobile communication system, for example, it is difficult to meet a strong demand for portability or compactness of the terminal.
The above non-linear wave receiving band-limit filters 123 and 126 are used to fixedly set respective pass-bands in advance. On the other hand, the non-linear modulated wave passes through a different band according to a type of system (analog communication system) such as AMPS, TACS, NAMPS, NTACS or the like so that it is difficult to versatilely use hardware to an applied non-linear modulated wave system.
In other words, in the above non-linear wave receiving band-limit filters 123 and 126, it is difficult to dynamically change a pass band. Accordingly, it is necessary to change or switch the filter to be used according to a required non-linear modulation system, besides it is difficult to reduce the cost or improve the reliability when the apparatus including the non-linear receiving filters is formed into an LSI (Large-Scale Integrated circuit).
Meanwhile, a two-mode receiving apparatus 100A shown in FIG. 13 has, although having structural elements basically similar to those of the above linear receiving circuit 110 shown in FIG. 12, non-linear wave receiving band-limit filters 126I and 126Q, and A/D converters 127I and 127Q on the output's side of the multipliers 112I and 112Q as a receiving system for a non-linear modulated wave, besides using the structural elements of the linear wave receiving circuit 110 (refer to reference numerals 111, 112I, 112Q, 114, 117 and 118), thereby reducing a scale of the circuit.
In the two-mode receiving apparatus 100A shown in FIG. 13, a digital processing unit 130A has a linear wave receiving process unit 131, a local oscillated frequency setting unit 132, a frequency correcting unit 133, a gain controlling unit 134 and a non-linear wave receiving process unit 136, basically similar to those of the above two-mode receiving apparatus 100 shown in FIG. 12.
The two-mode receiving apparatus 100A shown in FIG. 13 receives and demodulates a linear modulated wave in the same way as the above two-mode receiving apparatus 100 shown in FIG. 12 when receiving the linear modulated wave. When receiving a non-linear modulated wave, the two-mode receiving apparatus 100A shown in FIG. 13 performs an automatic gain control, then converts the non-linear modulated wave into baseband signals in two systems, as well as the above linear modulated wave.
However, in the above two-mode demodulating apparatus 100A shown in FIG. 13, the non-linear wave receiving band-limit filters 126I and 126Q are not variable. In addition, it is impossible to completely eliminate steady-state frequency deviation in the quasi-synchronous detection system so that it is difficult to perform a demodulating process corresponding to each of various non-linear modulated waves (particularly, NAMPS, NTACS, etc.).
Namely, although the above two-mode receiving apparatus 100A shown in FIG. 13 converts not only a linear wave modulated signal but also a non-linear wave modulated signal into baseband signals in two systems in the quasi-synchronous detection system, it is difficult to demodulate the signal with a high accuracy since frequency deviation in the non-linear modulated wave in a system such as NTACS, NAMPS or the like is particularly small.
In this case, an AFC (automatic frequency control) operation, which is not required originally in the non-linear receiving circuit, is required in order to eliminate the above steady-state frequency deviation, besides the A/D converters 116I, 116Q, 126I and 126Q are required to be provided separately for the linear modulated wave and the non-linear modulated wave, which causes an increase in power consumption.